ci: Replace .build-only-schedules template with multiple extension

`.build` and `.only-schedules` are orthogonal, and I want to use
`.only-schedules` together with `.cross-template` in future, which would
require creating more cross-product templates.

Avoid that by splitting `extends: .build-only-schedules` into
```
extends:
  - .build
  - .only-schedules
```

Multiple extends are supported by GitLab: https://docs.gitlab.com/ee/ci/yaml/#extends

This introduces no functional changes.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -26,20 +26,21 @@ variables:
   except:
     - tags
 
+# Some jobs take a long time and are unlikely to find failures (or will find
+# failures which are not merge-blockers to fix), so they’re executed on a weekly
+# schedule in order to save CI resources and speed up branch pipelines.
+.only-schedules:
+  only:
+    - schedules
+  except:
+    - tags
+
 .build:
   extends: .only-default
   before_script:
     - bash .gitlab-ci/show-execution-environment.sh
     - cp -r $HOME/subprojects/* subprojects/
 
-# Some jobs take a long time and are unlikely to find failures (or will find
-# failures which are not merge-blockers to fix), so they’re executed on a weekly
-# schedule in order to save CI resources and speed up branch pipelines.
-.build-only-schedules:
-  extends: .build
-  only:
-    - schedules
-
 style-check-diff:
   extends: .only-default
   image: $DEBIAN_IMAGE
@@ -139,7 +140,9 @@ debian-stable-x86_64:
       - "_build/${CI_JOB_NAME}-report.xml"
 
 installed-tests:
-  extends: .build-only-schedules
+  extends:
+    - .build
+    - .only-schedules
   image: $FEDORA_IMAGE
   stage: build
   needs: []
@@ -175,7 +178,9 @@ installed-tests:
       - "_build/installed-tests-report/"
 
 G_DISABLE_ASSERT:
-  extends: .build-only-schedules
+  extends:
+    - .build
+    - .only-schedules
   image: $FEDORA_IMAGE
   stage: build
   needs: []
@@ -202,7 +207,9 @@ G_DISABLE_ASSERT:
       - "_build/${CI_JOB_NAME}-report.xml"
 
 valgrind:
-  extends: .build-only-schedules
+  extends:
+    - .build
+    - .only-schedules
   image: $FEDORA_IMAGE
   stage: analysis
   needs: []
@@ -478,7 +485,9 @@ coverage:
   coverage: '/^\s+lines\.+:\s+([\d.]+\%)\s+/'
 
 scan-build:
-  extends: .build-only-schedules
+  extends:
+    - .build
+    - .only-schedules
   image: $FEDORA_IMAGE
   stage: analysis
   needs: []
@@ -502,7 +511,9 @@ scan-build:
       - "_scan_build/meson-logs/scanbuild"
 
 coverity:
-  extends: .build-only-schedules
+  extends:
+    - .build
+    - .only-schedules
   image: $COVERITY_IMAGE
   stage: analysis
   needs: []